In the mosquito Aedes aegypti, blood feeding activates vitellogenesis that involves yolk protein precursor (YPP) genes in an insect metabolic tissue, the fat body. Vitellogenesis is regulated by the 20-hydroxyecdysone (20E) regulatory hierarchy, in which the Ets-domain protein E74 is a key transcriptional regulator. The mosquito AaE74 gene encodes two isoforms-AaE74A and AaE74B. Both AaE74 isoforms are 20E-inducible early gene products. AaE74B reaches its maximal expression at 10(-7)M of 20E, while AaE74A requires 10(-6)M of 20E, a concentration at which the YPP genes reach their maximal induction level. In transfection assay, AaE74B is capable of activating a reporter construct containing E74-response elements, while expression of AaE74A has no effect on the basal levels of the reporter. The AaE74B binding activity is present in the fat body nuclei only during active vitellogenesis. Taken together, our findings demonstrate that AaE74B isoform plays the role of a transcriptional activator during vitellogenesis.